Add basic .xrdb/.Xresources import plugin

Adds basic support for importing .Xresources and .xrdb files as Theme
resources.

Examples of terminal color schemes in .xrdb format can be found here:
https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/xrdb

Examples of terminal themes in .Xresources format can be found here:
https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/Xresources

https://terminal.sexy also supports exporting themes is Xresources
format.

test/import_plugins/xrdb/Test2.Xresources

@@ -0,0 +1,36 @@
+! special
+*.foreground:   #c5c8c6
+*.background:   #1d1f21
+*.cursorColor:  #c5c8c6
+
+! black
+*.color0:       #282a2e
+*.color8:       #373b41
+
+! red
+*.color1:       #a54242
+*.color9:       #cc6666
+
+! green
+*.color2:       #8c9440
+*.color10:      #b5bd68
+
+! yellow
+*.color3:       #de935f
+*.color11:      #f0c674
+
+! blue
+*.color4:       #5f819d
+*.color12:      #81a2be
+
+! magenta
+*.color5:       #85678f
+*.color13:      #b294bb
+
+! cyan
+*.color6:       #5e8d87
+*.color14:      #8abeb7
+
+! white
+*.color7:       #707880
+*.color15:      #c5c8c6